There are three main types of radioactive decay: alpha, beta, and gamma. Alpha decay releases alpha particles, beta decay releases electrons, and gamma decay releases gamma rays. The half-life of a radioactive isotope is the amount of time it takes for half of the radioactive substance to decay, and is used to determine the age of fossils and rocks. In the examples given, calculations are shown for determining the amount of radioactive material remaining after a given number of half-lives based on the original amount and the half-life of the specific isotope.